Original SBD Dauntless Blueprints: Arrangement Drawing Issues

In some aircraft it is difficult to provide the precise value of overall length. One of them is the SBD Dauntless, because of its easily demountable spinner used in the first three variants (SBD-1…-3). Also the length of the Hamilton Standard Hydromatic spinner hub, used in the later SBD variants, can vary – especially in the restored aircraft. Thus, for verification of model kits or similar purposes I would suggest checking the distance between two easily distinguishable points: from the firewall to the tip of the tail cone. This dimension remains the same in all SBD variants. Preparing the fuselage blueprints for my model, I could determine this distance using the tail cone assembly drawing:

Figure 111-1 Using the marked stations to measure distance from the firewall

The key information is provided by the stations marked in this drawing: their names describe distances from the firewall. (You can read them yourself from the high-resolution version of this drawing). I could read that the fuselage tip cone starts at station 271.6875 (11/16) and ends after station 308. However, this drawing does not provide the closing dimension (from station 308 to the tip), thus I had to measure this distance. Assuming a quite wide tolerance (1%), I estimated it as 6.7”. This means that the distance from the firewall to this fuselage tip was about 114.7” (+/- 0.05”). The light bulb protruding from the tip is not documented in this drawing. From the photos I could estimate its length as 1”. However, the information provided in this blueprint forced me to follow the “Douglas convention” of skipping the light bulbs dimensions. I already noticed this convention in the case of the SBD wing span: it was distance between the bases of the running lights, located on the opposite wing tips.

Later I discovered that I could verify this estimated distance using certain dimensions from the general arrangement drawing (dwg no 5120284):

Figure 111-2 Verification of the same dimension

I marked these three partial dimensions in the figure above. I subtracted from the total length (399.25”) the distance from the spinner tip to the wing leading edge (at STA 66: 91.56”), then added the distance from this point to the firewall (9”). I was surprised when I obtained a quite different result: 316.69”. This is 2” more than I measured in the previous blueprint – I could not explain such a significant difference!

However, looking at this arrangement drawing, I found another dimensioning chain:

Figure 111-3 Another verification (in the same drawing)

First two dimensions (9” + 274.812”) describe position of the rudder hinge axis, while the third dimension describes distance from this axis to the rudder tip (29.875”). (This rudder chord length is confirmed by corresponding dimension in its assembly drawing no 5156459). Obtained result: 313.688” is 1” short of my measurement, but it does not include the small part of the fuselage tip behind the rudder:

Figure 111-4 Details of the second result

Fortunately, another source confirmed this result: the arrangement diagram from the SBD-6 maintenance manual:

Figure 111-5 Checking the same dimension in the another arrangement diagram

Although the aircraft total length here is slightly different from dwg 5120284 (33’ 0.1” vs 33’ 0.25”), all three dimensions related to the rudder size and location are identical. (Just their fractional parts are expressed using decimals). This arrangement drawing is more detailed: at the tail tip you can see 1” dimension related to the remaining distance from the rudder tip to the running light base. Thus, the distance from the firewall to the running light base at the tail tip (314.687”) agrees with my initial measurement (314.7”).

Comparing these two versions of the arrangement diagram I found some other simplifications in dwg 5120284. It seems that it was drawn by a draftsman who skipped many detailed dimensions, and truncated the less important fractional parts from the remaining ones:

Figure 111-6 Truncated fractions in drawing 5120284 dimensions

Fortunately, the main dimensions like the overall span of the stabilizer are identical in both drawings.

Frankly speaking, after finding these errors I decided that I cannot trust dimensions from drawing 5120284, if they are not confirmed in other blueprints. This conclusion seems to be quite paranoid, but it looks that somebody in Douglas accepted this blueprint without proper verification.

Ultimately, for the SBD scale plans and model kits I propose following length checks:

Figure 111-7 Length verification proposal

Both distances are identical in all Dauntless versions.

I do not suggest checking the overall wing span, because the value used in all general drawings (including the manufacturer arrangement diagram): 41’ 6.37” is wrong due to conceptual mistake. (I described this error at the end of in this post). The real SBD wing span was 41’ 3.2”, but I doubt that any drawing/model kit fits this dimension. Thus I suggest measurement of two other spans, which are confirmed in all assembly blueprints:

Figure 111-8 Span verification proposal

I suppose that you can trust more the model kit/scale plan that fits these four dimensions. The other distances can vary, due to wrong wing span, provided by Douglas itself, or the wrong fuselage length specified in the BuAer SBD-1, SBD-2 and SBD-3/-4 performance reports.  (These lengths were repeated in all publications about his aircraft. In the result – for the SBD-3 both: the length and the span were wrong in all sources that I saw. See this and this post for more details).

Update (2021-03-05): drawing 5120284 is OK: I discovered that I made an error in Figure 111-2, recalculating the overall length from feet to inches. 33′ = 396″, while in my calculations I used 399″. Subtracting the dimensions marked in Figure 111-2 from the overall length gives the same result as in Figure 111-3: 113 5/16″